Stone Age people chewed birch bard tar,which forms when the bark is heated.后面为什么用现在时不明白,请指教!

Stone Age people chewed birch bard tar,which forms when the bark is heated.后面为什么用现在时不明白,请指教!
Stone Age people chewed birch bard tar,which forms when the bark is heated.
后面为什么用现在时不明白,请指教!

Stone Age people chewed birch bard tar,which forms when the bark is heated.后面为什么用现在时不明白,请指教!
石器时代的人咀嚼白桦焦油,白桦焦油是由树皮被加热所形成的.

which forms when the bark is heated. 不是强调过去的行为动作,而是一种常理或真理的时候,都用一般现在时.同“太阳从东边升起”一样的道理.

咀嚼是发生在过去的动作所以用过去式。但是加热形成油却不是过去的动作是一种自然规律，所以用现在时。